Two stage melt production of high molecular weight polybenzimidazoles with phosphorous catalyst

ABSTRACT

This invention provides a two stage melt polymerization process for the production of an improved type of high molecular weight polybenzimidazole, which involves the use of an arylphosphinic acid or arylphosphonic acid polymerization catalyst. 
     The process provides poly-2,2&#39;-(m-phenylene)-5,5&#39;-bibenzimidazole of improved color and plugging value, and is further characterized by an inherent viscosity of at least about 0.7 dl/g, and a weight average molecular weight of at least about 100,000.

This is a division of application Ser. No. 461,888, filed Jan. 28, 1983,now U.S. Pat. No. 4,452,972.

BACKGROUND OF THE INVENTION

Typical processes for preparing polybenzimidazoles are described inJournal of Polymer Science, 50, 511 (1961), and in various U.S. patents.

U.S. Pat. No. 3,174,947 (U.S. Pat. No. Re. 26,065) describes a method ofpreparing high molecular weight aromatic polybenzimidazoles by meltpolymerizing an aromatic tetraamine and a diphenyl ester or an anhydrideof an aromatic dicarboxylic acid at an elevated temperature andthereafter further polymerizing the product of the melt polymerizationin the solid state. According to this process, in order to producepolymers of sufficiently high molecular weight to be suitable forpractical use it is necessary to finely pulverize the product of themelt polymerization prior to polymerization in the solid state and toconduct the solid state polymerization at an elevated temperature undera reduced pressure of less than 0.5 mm Hg or at an elevated temperatureand in an inert gas stream over a prolonged period of time. Thus, theprocess requires several complicated operations. In addition, since thereaction is conducted over a long period of time at an elevatedtemperature, it tends to form insoluble and infusible polymers.

U.S. Pat. No. 3,313,783 describes a process adapted to overcome theabove-mentioned deficiencies which process involves the solutionpolymerization of an inorganic acid salt of an aromatic tetraamine and adicarboxylic acid or a derivative thereof with heat in polyphosphoricacid. After completion of the reaction the polymer product is separatedby pouring the reaction mixture in the form of a polyphosphoric acidsolution into a large quantity of water. However, this separationprocedure is complicated and it is difficult to recover and reuse thepolyphosphoric acid.

Another process for producing polybenzimidazoles is described in U.S.Pat. No. 3,509,108. In the process the monomers are initially reacted ina melt phase polymerization at a temperature above 200° C. and apressure above 50 psi. The reaction product is then heated in a solidstate polymerization at a temperature above 300° C. to yield the finalaromatic polybenzimidazole product. The process requires that theinitial reaction be conducted at a pressure above 50 psi (preferably,between 300-600 psi) in order to control the foaming encountered duringthe polymerization.

U.S. Pat. No. 3,555,389 describes a two stage process for the productionof aromatic polybenzimidazoles. The monomers are heated at a temperatureabove 170° C. in a first stage melt polymerization zone until a foamedprepolymer is formed. The foamed prepolymer is cooled, pulverized, andintroduced into a second stage polymerization zone where it is heated inthe presence of phenol to yield a polybenzimidazole polymer product. Aswith the process of U.S. Pat. No. Re. 26,065, this process involvesmultiple operations and tends to form insoluble polymers.

U.S. Pat. No. 3,433,772 describes a two stage polymerization process forthe production of aromatic polybenzimidazoles which utilize an organicadditive, such as an alkane having 11-18 carbon atoms or apolycarbocyclic hydrocarbon, in order to control foaming during thefirst stage.

Other U.S. patents relating to one stage and two stage production ofpolybenzimidazoles include U.S. Pat. Nos. 3,408,336; 3,549,603;3,708,439; 4,154,919; and 4,312,976; all patents enumerated hereinincorporated by reference.

Technical Report AFML-TR-73-22 (Air Force Material Laboratory,Wright-Patterson AFB, Ohio) describes the production ofpolybenzimidazole from tetraaminobiphenyl and diphenyl isophthalate inthe presence of various catalysts such as ammonium chloride,hydrochloric acid, p-toluenesulfonic acid, phosphoric acid,triphenylphosphate and boron trifluorideetherate.

In the Technical Report (pages 26-27) the data indicate thatphosphorus-containing catalysts such as phosphoric acid and triphenylphosphate are effective for increasing the inherent viscosity of apolybenzimidazole resin. However, there is formation of gel andinsoluble black specks which tend to affect adversely the Plugging Valueproperty of the polybenzimidazole products.

There remains a need for an improved method of producingpolybenzimidazole which overcomes the various disadvantages of the priorart procedures, and which method yields an improved type ofpolybenzimidazole product particularly suitable for the formation offibers having a high melting point and a high degree of thermalstability.

Accordingly, it is an object of the present invention to provide animproved two stage polymerization process for the production of highmolecular weight polybenzimidazole in the presence of aphosphorus-containing catalyst.

It is another object of this invention to provide a high molecularweight polybenzimidazole which is characterized by an improved pluggingvalue and an improved color specification.

Other objects and advantages of the present invention shall becomeapparent from the accompanying description and examples.

DESCRIPTION OF THE INVENTION

One or more objects of the present invention are accomplished by theprovision of a two stage polymerization process for the production ofhigh molecular weight polybenzimidazole from a mixture of (1) at leastone aromatic tetraamine containing two groups of amine substituents,said amine substituents in each group being in an ortho positionrelative to each other, and (2) at least one dicarboxylate ester, whichcomprises heating the mixture above its melting temperature in a firststage melt polymerization zone in the presence of a phosporus-containingpolymerization catalyst selected from arylphosphonic acids andarylphosphinic acids, to provide a foamed prepolymer; and heating theprepolymer in a second stage solid state polymerization zone at atemperature above about 250° C. to produce high molecular weightpolybenzimidazole product.

In a further embodiment, the present invention provides a two stagepolymerization process for the production of high molecular weightpolybenzimidazole from at least one monomeric aromatic reactant having apair of amine substituents in an ortho position relative to each otherand a carboxylate ester group positioned on an aromatic nucleus, whichcomprises heating the reactant above its melting temperature in a firststage melt polymerization zone in the presence of aphosphorus-containing polymerization catalyst selected fromarylphosphonic acids and arylphosphinic acids, to provide a foamedprepolymer; and heating the prepolymer in a second stage solid statepolymerization zone at a temperature above about 250° C. to produce highmolecular weight polybenzimidazole product.

Polybenzimidazoles are a known class of heterocyclic polymers whichconsist essentially of recurring units of the following Formulas I andII. Formula I is: ##STR1## where R is a tetravalent aromatic nucleuswith the nitrogen atoms forming the benzimidazole rings being pairedupon adjacent carbon atoms, i.e., ortho carbon atoms, of the aromaticnucleus, and R¹ is a member of the class consisting of an aromatic ring;an alkylene group (preferably having 4 to 8 carbon atoms); and aheterocyclic ring such as pyridine, pyrazine, furan, quinoline,thiophene, and pyran.

Formula II is: ##STR2## where Z is an aromatic nucleus having thenitrogen atoms forming the benzimidazole ring paired upon ajacent carbonatoms of the aromatic nucleus.

Any of the above polybenzimidazoles represented by the Formulas I and IIcan be prepared by the process of the present invention.

The following generalized equation illustrates the condensation reactionwhich occurs in forming the polybenzimidazoles having the recurringunits of Formula I: ##STR3##

Such polybenzimidazoles are produced by the reaction of a mixture of (1)at least one aromatic tetraamine containing two groups of aminesubstituents, the amine substituents in each group being in an orthoposition relative to each other, and (2) at least one dicarboxylateester in which R² in the compound shown is a substituent selected fromaliphatic, alicyclic and aromatic groups.

It is advantageous to use as the aromatic tetraamines compounds such asthose illustrated below: ##STR4## where X represents --O--, --S--, --SO₂--, ##STR5## or a lower alkyl group, such as --CH₂ --, --(CH₂)₂ --, or--C(CH₃)₂ --. Such aromatic tetraamines include1,2,4,5-tetraaminobenzene; 1,2,5,6-tetraaminonaphthalene;2,3,6,7-tetraaminonaphthalene; 3,3',4,4'-tetraaminodiphenyl ether;3,3',4,4'-tetraaminobiphenyl; 3,3',4,4'-tetraaminodiphenylmethane;3,3',4,4'-tetraamino-1,2-diphenylethane;3,3',4,4'-tetraamino-2,2-diphenylpropane;3,3',4,4'-tetraaminodiphenylthioether; and3,3',4,4'-tetraaminodiphenylsulfone. The preferred aromatic tetraamineis 3,3',4,4'-tetraaminobiphenyl.

The dicarboxylate esters which are suitable for use in the production ofpolybenzimidazoles by the process of the present invention includearomatic dicarboxylate esters; aliphatic dicarboxylate esters(preferably, those having 4 to 8 carbon atoms in the carboxylic acidstructure); and heterocyclic dicarboxylate esters wherein the estergroups are substituents upon carbon atoms in a ring compound such aspyridine, pyrazine, furan, quinoline, thiophene, and pyran.

The preferred dicarboxylate esters are aromatic dicarboxylate esterssuch as those illustrated below: ##STR6## where X and R² are as definedabove. Typical dicarboxylate esters include diesters of terephthalicacid; isophthalic acid; 4,4'-biphenyldicarboxylic acid;1,4-naphthalenedicarboxylic acid; 1,6-naphthalenedicarboxylic acid;2,6-naphthalenedicarboxylic acid; 4,4'-diphenyletherdicarboxylic acid;4,4'-diphenylmethanedicarboxylic acid; 4,4'-diphenylsulfonedicarboxylicacid; and 4,4'-diphenylthioetherdicarboxylic acid. Diphenyl isophthalateis the preferred dicarboxylate ester for use in the present inventionprocess.

It is preferred to employ the dicarboxylate ester in a ratio of about 1mole per mole of aromatic tetraamine. However, in order to obtain aproduct having an optimum molecular weight, it can be advantageous toemploy an excess (e.g., 0.25 mole percent) of either the dicarboxylateester or the tetraamine monomeric reactant.

Examples of polybenzimidazoles which have the recurring structure ofFormula I and which may be prepared according to the process of thepresent invention include:

poly-2,2'-(m-phenylene)-5,5'-dibenzimidazole);

poly-2,2'-(pyridylene-3",5")-5,5'-bibenzimidazole;

poly-2,2'-(furylene-2",5")-5,5'-bibenzimidazole;

poly-2,2'-(naphthalene-1",6")-5,5'-bibenzimidazole;

poly-2,2'-(biphenylene-4",4")-5,5'-bibenzimidazole;

poly-2,2'-amylene-5,5'-bibenzimidazole;

poly-2,2'-octamethylene-5,5'-bibenzimidazole;

poly-2,6-(m-phenylene)-diimidazobenzene;

poly-2,2'-cyclohexenyl-5,5'-bibenzimidazole;

poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)ether;

poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)sulfide;

poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)sulfone;

poly-2,2'-(m-phenylene)-5,5'-di(benzimidazole)methane;

poly-2',2"-(m-phenylene)-5',5"-di(benzimidazole)propane-2,2; and

poly-2,2'-(m-phenylene)-5',5"-di(benzimidazole)ethylene-1,2.

The particukarly preferred polybenzimidazole of Formula I prepared bythe process of the present invention ispoly-2,2'-(m-phenylene)-5,5'-bibenzimidazole as characterized by therecurring unit: ##STR7## The illustrated polymer can be prepared inaccordance with the present invention process by the reaction of3,3',4,4'-tetraaminobiphenyl with diphenyl isophthalate.

The polybenzimidazoles having the recurring units of Formula II can beprepared by the autocondensation of at least one aromatic compoundhaving a pair of amine substituents in an ortho position relative toeach other and a carboxylate ester group positioned upon an aromaticnucleus (referred to hereinafter as the diaminocarboxylate esterreactant). Examples of such compounds are esters of diaminocarboxylicacids which include 3,4-diaminobenzoic acid;5,6-diaminonaphthalene-1-carboxylic acid;5,6-diaminonaphthalene-2-carboxylic acid;6,7-diaminophthalene-1-carboxylic acid;6,7-diaminonaphthalene-2-carboxylic acid, and the like. A preferredcompound is 4-phenoxycarbonyl-3',4'-diaminodiphenyl ether: ##STR8##

According to the present invention process, high molecular weightpolybenzimidazole is produced by reacting the monomeric reactant orreactants described above. The preferred reactants are a mixture of atleast one aromatic tetraamine and at least one dicarboxylate ester, asdefined above, with 3,3',4,4'-tetraaminobiphenyl and diphenylisophthalate being especially preferred. In the first stage meltpolymerization zone, the monomeric reactant or reactants are polymerizedat a temperature above the melting temperature of the monomeric reactantor reactants in the presence of arylphosphonic acid or arylphosphinicacid catalyst to provide a foamed prepolymer.

The first stage melt polymerization reaction usually is conducted at atemperature of at least about 170° C., and preferably at a temperaturewithin the range between about 200°-300° C. In a typical procedure asdescribed in U.S. Pat. No. 3,549,603, the first stage meltpolymerization reaction is conducted in a substantially molecularoxygen-free atmosphere for a period of time sufficient to form aprepolymer having an inherent viscosity of at least 0.1 (e.g., 0.13-0.3)dl/g. The average reaction time will vary between about 0.5-3 hours.

Normally a pressure between about 15-50 psi, preferably atmosphericpressure with a flow of nitrogen, is employed in the first stage meltpolymerization zone. In a typical run an open polymerization system isutilized which is provided with a distillation column to remove thewater and alcohol which are produced as byproducts of thecondensation-polymerization reaction.

At the conclusion of the first stage melt polymerization reaction, thefoamed prepolymer product is cooled and pulverized, and the resultantprepolymer powder is introduced into the second stage solid statepolymerization zone.

In this zone, the prepolymer powder is heated at a temperature aboveabout 250° C., preferably in the range between about 320°-420° C. atambient pressure for a period of time sufficient to yield a highmolecular weight polybenzimidazole product.

The second stage reaction is conducted in a substantially oxygen-freeatmosphere as with the first stage reaction. For example, an inert gassuch as nitrogen or argon can be continuously passed through thereaction zone during the polymerization. The inert gas employed shouldbe substantially oxygen-free, i.e., it should contain less than about 8ppm of molecular oxygen. The inert gas is introduced into the reactionzone at a rate of flow measured at standard conditions, i.e.,atmospheric pressure and temperature, within the range of about 1-200percent of volume of the reaction zone per minute. The inert gas can bepassed into the polymerization reaction zone at room temperature or itcan be preheated to the reaction temperature.

The second stage polymerization process is conducted for a period oftime sufficient to produce a high molecular weight polybenzimidazoleproduct. The inherent viscosity of the polybenzimidazole product is anindication of its molecular weight. The high molecular weightpolybenzimidazoles produced by the invention process exhibit an inherentviscosity of at least approximately 0.6 dl/g when measured at aconcentration of 0.4 g of the polymer in 100 ml of 97 percent H₂ SO₄ at25° C. Preferably, the polymer exhibits an inherent viscosity of atleast approximately 0.7 dl/g. In order to obtain a high molecular weightpolybenzimidazole product, the polymerization in the second stage zoneis conducted for a reaction period of at least about 0.5 hour, e.g., areaction period between about 1-4 hours.

In the invention two stage polymerization system, the polymerizationcatalyst is employed in a quantity between about 0.01-5 weight percent,based on the total weight of monomeric material being reacted. In atypical ttarting mixture the weight of polymerization catalyst employedis in the range between about 0.1-1.0 percent, based on the total weightof monomeric material introduced into the first stage meltpolymerization zone.

An important aspect of the present invention is the use of a specificclass of phosphorus-containing polymerization catalysts. Illustrative ofpreferred catalysts are arylphosphonic acid and arylphosphinic acidscorresponding to the formulae: ##STR9## where M is an aryl substituent,and M¹ is a substituent selected from hydrogen, alkyl and aryl groups.Suitable catalysts include phenylphosphonic acid, tolylphosphonic acid,naphthylphosphonic acid, phenylphosphinic acid, naphthylphosphinic acid,butyl-phenylphosphinic acid, diphenylphosphinic acid, and the like. Mand M¹ as defined above can contain non-interfering heteroatoms such asoxygen, sulfur, nitrogen and halogen.

A present invention phosphorus-containing polymerization catalyst issuperior to prior art catalysts, such as phosphoric acid or triphenylphosphate as previously described above, in that there is little or noformation of gel and/or insoluble black specks, the presence of whichare detrimental to the Plugging Value property.

An important advantage derived from the practice of the presentinvention two stage polymerization process is the provision of a highmolecular weight polybenzimidazole which has a novel combination ofdesirable properties. Thus, under optimal operating conditions thepresent invention process provides polybenzimidazole which ischaracterized by properties comprising an Inherent Viscosity of at leastabout 0.7 dl/g when measured at a concentration of 0.4 g of saidpolybenzimidazole in 100 ml of 97 percent sulfuric acid at 25° C.; aWeight Average Molecular Weight of at least about 100,000; a PluggingValue of greater than about 0.5; a Gardner Color of less than about 10;and which polybenzimidazole is capable of forming a 15 weight percentsolution when heated at 240° C. for two hours in dimethylacetamidesolvent containing 2 weight percent of lithium chloride.

Polybenzimidazole (PBI) molecular weight constants referred to hereinare determined by employing a Waters 201 instrument with two poroussilica columns (Waters μ-Bondagel E-linears) at the concentration of 0.2g wt/vol in DMAc containing 2% LiCl. The calculations are based on acalibration curve obtained with eleven polystyrene molecular weightstandards with the range between 2100 and 1,800,000, and the PBImolecular weight corresponds to the polystyrene equivalent molecularweight.

Number Average Molecular Weight (Mm) and Weight Average Molecular Weight(Mw) and Molecular Weight Distribution are related as follows:

    MWD=Mw/Mn

The double logarithm of Intrinsic Viscosity (97% H₂ SO₄) versus WeightAverage Molecular Weight (DMAc, 2% LiCl) can be plotted, and theMark-Houwink equation for PBI molecular weight can be derived from theplot and expressed as

    .sup.[η] H.sub.2 SO.sub.4 =1.35326×10.sup.-4 M.sup.0.73287

The relationship of Intrinsic Viscosity to Inherent Viscosity of PBI in97% sulfuric acid can be expressed as:

    [η]=1.0585·ηinh

By using these expressions, the molecular weight of PBI polymers can beestimated from intrinsic or inherent viscosities in a reasonableagreement with the experimental values.

Plugging Value (P.V.) referred to herein is a measure of solutionfilterability, and is determined from the filtration rate through a finefilter paper. The PBI polymer to be evaluated is dissolved in 97.0±0.1%sulfuric acid at a known concentration of 5 to 7%. The solution isprepared by weighing the desired quantity of sulfuric acid (e.g., 95grams) into a 250 ml glass-stoppered Erlenmeyer flask, and then weighingthe desired quantity of polymer (e.g., 5 grams) with the flask. Thepolymer is dissolved by shaking on a wrist-action shaker for a minimumof two to three days depending upon the Intrinsic Viscosity (I.V.) level(i.e., longer for higher I.V.'s). The filtration apparatus consists of a2" stainless steel pressure filter (Gelman Instrument Co.), Model 4240,and Gelman Type A, 2" glass filter papers. The solution after shaking ispoured into the filter chamber and one atmosphere of pressure from anitrogen cylinder is applied. Zero time is taken as the first dropreaches the receiver located on a balance below the filtration unit. Theweight of the solution that passes through the filter is recorded withrespect to time. Data is continuously generated until either the filteris emptied or the filtration rate is very slow as a result of plugging.

Calculation of the Plugging Value is as follows:

1. Each time in minutes is divided by the corresponding weight in gramsof solution (total) through the filter at that time.

2. The obtained values are plotted against time. The resulting graphapproximates a straight line. The initial curvature is disregarded aswell as the last few points if curvature would again exist.

3. The reciprocal slope of the graph represents the theoretical weightin grams of solution that would pass through the filter in infinite timein order to plug it. This is designated as W∞. ##EQU1##

Polybenzimidazole Gardner Color referred to herein is a value which isdetermined relative to the Gardner Color scale (Gardner Laboratory Inc.,Bethesda, Md.).

The Gardner Color scale consists of a gradation of twenty color shadesranging from water-white(1) to dark brown(20). In accordance with thepresent invention, polybenzimidazole is provided which has a GardnerColor of less than about 10. A typical commercial type polybenzimidazolefiber has a Gardner Color of about 14.

The following Examples are further illustrative of the presentinvention. The reactants and other specific ingredients and conditionsare presented as being typical, and various modifications can be devisedin view of the foregoing disclosure within the scope of the invention.

EXAMPLE I

Into a three-necked flask equipped with a nitrogen inlet and outlet,mechanical stirrer and a condenser are placed 23.333 g (0.1089 mole) of3,3',4,4'-tetraaminobiphenyl, 34.67 g (0.1089 mole) ofdiphenylisophthalate and 0.3 g of diphenylphosphinic acid. The flask isdegassed and then filled with nitrogen. The degassing is repeated atleast three times. The mixture is heated rapidly with stirring to 225°C. The stirring is stopped. The temperature of the reaction mixture isthen raised to 270° C. and held at that temperature for 1.5 hours. Theresulting product is cooled to room temperature and then is ground.

The ground prepolymer is placed in a flask and after the degassing stepis repeated the prepolymer is heated at 360° C. for one hour. Theresulting polybenzimidazole exhibits a weight average molecular weightof 201,700 with a molecular weight distribution of 2.91 and an inherentviscosity of 1.00 dl/g when measured in a concentration of 0.4 g of thepolybenzimidazole in 100 ml of 97% sulfuric acid.

Similar results are obtained when the catalyst is phenylphosphonic acidor diphenylphosphinic acid.

A polybenzimidazole polymer as prepared above is further characterizedby a Plugging Value of greater than about 0.5, and a Gardner color ofless than about 10.

EXAMPLE II

The polybenzimidazole of Example I is prepared according to theprocedure described in Example I, but without the inclusion of acatalyst. The inherent viscosity of the polybenzimidazole thus producedis 0.68 dl/g when measured in a concentration of 0.4 g of thepolybenzimidazole in 100 ml of 97% sulfuric acid at 25° C. This polymerhas a weight average molecular weight of 113,000 and molecular weightdistribution of 2.56.

The Plugging Value and Gardner Color of this polybenzimidazole areinferior to those obtained by the Example I invention process.

What is claimed is:
 1. A two stage polymerization process for theproduction of high molecular weight polybenzimidazole from at least onemonomeric aromatic reactant having a pair of amine substituents in anortho position relative to each other and a carboxylate ester grouppositioned on an aromatic nucleus, which comprises heating the reactantabove the reactant melting temperature in a first stage meltpolymerization zone in contact with a phosphorus-containingpolymerization catalyst selected from arylphosphonic acids orarylphosphinic acids, to provide a foamed prepolymer; and heating theprepolymer in a second stage solid state polymerization zone at atemperature above about 250° C. to produce high molecular weightpolybenzimidazole product.
 2. A process in accordance with claim 1wherein the monomeric aromatic reactant is4-phenoxycarbonyl-3',4'-diaminodiphenyl ether.